Hi

If you remain airside then a transit visa is not required at all.

Just for information and for the information of anyone else reading the thread, transit visas are only required if you need to enter landside; i.e. actually 'entering' the country. A hypothetical example is where the onward flight doesn't leave until the next day and you'd need to stay in an airport/nearby hotel overnight. Being a Ukrainian national then technically she would require a Visa to enter the UK. However even in this situation entry can be granted without a visa - a Transit Without Visa. The conditions are only until the end of the next day, as long as entry is assured to the onward country (the Green Card ensures this) and there is a confirmed flight booking (which she'd already have). She'd be granted entry for one day to enable her to continue her onward journey.

Hope that helps.